Solar Wind Charge Exchange Contamination

Figure 3: XMM-Newton MOS SWCX contaminated spectrum compared to an uncontaminated spectrum; both ObsIDs are from the Hubble Deep Field North direction that were analyzed in Snowden, Collier, & Kuntz (2004). Note the very strong excess O VIII emission, as well as other line emission, in the SWCX spectrum. These are also lines of interest as diagnostics of astrophysical plasmas.

Solar wind charge exchange (SWCX) emission can contribute significantly to the observed flux below 1.5 keV (see Figure 3), as shown in Snowden, Collier, & Kuntz (2004). SWCX emission may occur both near the Earth, in the Earth's magnetosheath, where the solar wind interacts with exospheric material (e.g., Snowden et al. 2009), or distributed throughout the solar system where the solar wind interacts with interstellar neutrals (hydrogen and helium) flowing through the solar system (e.g., Koutroumpa et al. 2009). Further information can be found in Kuntz (2019).

The X-ray spectrum of the SWCX in the XMM-Newton band is dominated by line emission from ions that are also of astrophysical interest, e.g., O VII and O VIII. The magnitude of the emission is strongly variable, as are the line ratios that follow the flux, abundances, and ionization states of the solar wind. Thus, the use of standard line ratio plasma diagnostics, at least for diffuse emission filling the field of view, is extraordinarily sensitive to the amount of SWCX emission.

Carter & Sembay (2008) have published a method for identifying observations affected by SWCX emission based on the variation of the full FOV light curve for $E<1.35$ keV. This method is sensitive to the magnetospheric SWCX, but only if the observation is sufficiently long. The time scales for variation of the heliospheric SWCX are too long to allow a similar identification.

The XMM-Newton GOF has created a “Trend” data base[*] and associated software which can identify observations which have a greater likelihood of being affected by magnetospheric SWCX emission based on both observation geometry and solar wind flux.

We note that reliance upon the solar wind flux, as measured by upstream monitors such as ACE, Wind, or DSCOVR has significant uncertainties for reasons discussed at length by Kuntz (2019). Further, abundance data has become increasingly sparse over the last several years. Thus, it is now more difficult to assess the likelihood of significant SWCX contamination.